1. Field of Endeavor
The present invention refers to the area of power plant technology. It relates to a method for operating a combined cycle power plant, and to a combined cycle power plant useful for carrying out the method.
2. Brief Description of the Related Art
The present invention relates to a combined cycle power plant, such as is shown in a schematic manner as an example and greatly simplified in FIG. 1. The combined cycle power plant 10 of FIG. 1 includes a gas turbine installation 11 as well as a water-steam cycle 21 which is connected thereto.
The gas turbine installation 11 includes, in its simplest form, a compressor 13, which sucks in air via an air inlet 12 and compresses it and outputs the compressed air to a combustion chamber 14 downstream, where it is used to burn a fuel 15. The hot gas generated in the combustion chamber is expanded in a turbine 16 downstream under operating load (in the case of sequential combustion, several combustion chambers and turbines can also be present). The compressor 13 and the turbine 16 are usually arranged on a common rotor shaft which drives a generator 17 to generate electric energy. To cool the parts of the turbine 16 that are highly loaded thermally, air compressed at the compressor 13 (in the example in FIG. 1 at two different positions) is drawn off, cooled in associated coolers 18 or 19 and supplied to the turbine 16. The coolers 18, 19 are in the majority of cases realized as OTC coolers (Once Through Cooler) and are fed with water.
The exhaust gas 20 emerging from the turbine 16 is sent through a waste heat steam generator 24 which is arranged in the water-steam cycle 21 and there brings about the evaporation and super-heating of the water circulating in the water-steam cycle 21. The steam generated in the waste heat steam generator 24 is expanded in a steam turbine 23 under operating load and drives a generator 22 which is connected to the steam turbine 23. The steam turbine 23 can also be coupled directly to the gas turbine, a common generator being driven. The steam emerging from the steam turbine 23 is condensed in a condenser 25 and passes as condensation into a feedwater tank 26, from where a feedwater pump 27 conveys the water again to the waste heat steam generator 24 thus closing the cycle.
During cooling of the compressed air, drawn off at the compressor 13, in the coolers 18 and 19, the water allocated to the coolers 18, 19 is evaporated. If, instead of the combined cycle, the combined cycle power plant 10 is operated only as a simple cycle, where only the gas turbine installation 11 is operating and where the water-steam cycle 21 is not operating, the steam arising in the coolers 18, 19 has to be discharged in some way or other. This occurs up to now either by it being blown off into the atmosphere (e.g., by a secondary stack provided for the simple cycle), or by it being condensed in a condenser and being returned into the cycle. However, it can also be injected into the gas turbine to increase performance.
It would, however, be desirable to use the steam arising during the cooling of the cooling air to better effect. In particular, the transition from simple cycle operation to combined cycle operation might be considered for this when installation parts of the water-steam cycle which have been cooled (over several days) have to be preheated in order to reduce thermal stresses and/or to accelerate the start-up process.
U.S. Pat. No. 5,473,898 A has made known, when starting the combined cycle in a combined cycle power plant, preheating the steam turbine and the waste heat steam generator with hot air which is removed from the compressor. In this case, cooling the compressed air for cooling purposes does not take place.
U.S. Patent App. Pub. No. 2009/0301078 A1 proposes integrating the waste heat which occurs when cooling compressed air into the water-steam cycle of a combined cycle power plant via the waste heat steam generator. This type of operation, however, is only possible if the installation is already operating in the combined cycle.